An hour's nap = a night's sleep

An hour's nap = a night's sleep

Winston Churchill swore by a mid-afternoon siesta, while Margaret Thatcher famously maintained that she needed only four hours of sleep a night to run the country.

Tony Blair gets by on less than six.

Now scientists have discovered that an hour-long power nap can be as beneficial as a whole night's sleep.

But only if you dream while you're napping.

Researchers in the U.S. found that volunteers who dozed off during the day performed far better afterwards than those who stayed awake.

But napping only works when it includes two kinds of sleep - slow wave sleep and rapid eye movement sleep, according to the psychologists at Harvard University in Cambridge, Massachusetts.

The two kinds of sleep are characterised by different brainwave patterns.

Most dreaming occurs during REM sleep, when a person's eyes flick back and forth rapidly. Volunteers in the study were given a visual learning task - which involved identifying the position of bars on a screen - at 9am, 7pm, and 9am the following morning.

Those who were not allowed to nap during the day saw their performance decline when tested in the evening.

But those who napped for an hour - with both kinds of sleep - did significantly better than the no-nappers at 7pm.

Nappers who never entered REM sleep showed no improvement.

The scientists, led by Dr Sara Mednick, wrote in the journal Nature Neuroscience: "From the perspective of behavioural improvement, a nap is as good as a night of sleep for learning on this perceptual task."

Volunteers who took a 90-minute nap on the first day were 50 per cent better at remembering the task 24 hours later than the no-nappers.

"Indeed, 24-hour improvement in the nap group was as great as that previously reported after two nights of sleep," the researchers said.

More studies will be needed to determine the full benefits of napping.

Churchill's siestas, usually followed by a bath, were credited with giving him the stamina to work through the night until 4am during the Second World War.

Among other prominent figures from history, Napoleon Bonaparte advocated 'six hours of sleep for a man, seven for a woman and eight for a fool'. Leonardo da Vinci was also not a big sleeper.

Among modern high achievers, yachtswoman Ellen MacArthur revealed that she needed very little sleep to function at record-breaking efficiency.

During her time sailing Kingfisher around the world, Miss MacArthur never slept for longer than two hours.

As for Baroness Thatcher, it seems she may have needed some help to get by on four hours a night.

Her former assistant has revealed that the then Prime Minister relied on vitamin B 12 injections to maintain her energy levels throughout the day.